Biology Chapter 12

 Performed the first major experiment that led to the discovery of DNA as the genetic material Griffith

 Identified the molecule that transformed the R strain of bacteria into the S strain  Concluded that when the S cells were killed, DNA was released  R bacteria incorporated this DNA into their cells and changed into S cells. Avery

 Used radioactive labeling to trace the DNA and protein  Concluded that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses Hershey and Chase

 Nucleotides  Consist of a five-carbon sugar, a phosphate group, and a nitrogenous base DNA Structure

 Chargaff’s rule: C = G and T = A Chargaff

 X-ray diffraction data helped solve the structure of DNA  Indicated that DNA was a double helix X-ray Diffraction

 Built a model of the double helix that conformed to the others’ research 1. two outside strands consist of alternating deoxyribose and phosphate 2. cytosine and guanine bases pair to each other by three hydrogen bonds 3. thymine and adenine bases pair to each other by two hydrogen bonds Watson and Crick

 DNA often is compared to a twisted ladder.  Rails of the ladder are represented by the alternating deoxyribose and phosphate.  The pairs of bases (cytosine–guanine or thymine– adenine) form the steps. DNA Structure

 On the top rail, the strand is said to be oriented 5 ′ to 3 ′.  The strand on the bottom runs in the opposite direction and is oriented 3 ′ to 5 ′. Orientation

 DNA coils around histones to form nucleosomes, which coil to form chromatin fibers.  The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis. Chromosome Structure

 Parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA. Semiconservative Replication

 DNA helicase, an enzyme, is responsible for unwinding and unzipping the double helix.  RNA primase adds a short segment of RNA, called an RNA primer, on each DNA strand. Unwinding

 DNA polymerase continues adding appropriate nucleotides to the chain by adding to the 3 ′ end of the new DNA strand. Base pairing

 One strand is called the leading strand and is elongated as the DNA unwinds.  The other strand of DNA, called the lagging strand, elongates away from the replication fork.  The lagging strand is synthesized discontinuously into small segments, called Okazaki fragments. 

 DNA polymerase removes the RNA primer and fills in the place with DNA nucleotides.  DNA ligase links the two sections. Joining

 Eukaryotic DNA unwinds in multiple areas as DNA is replicated.  In prokaryotes, the circular DNA strand is opened at one origin of replication. Comparing DNA Replication in Eukaryotes and Prokaryotes

 RNA  Contains the sugar ribose and the base uracil  Usually is single stranded Central Dogma

 Messenger RNA (mRNA)  Long strands of RNA nucleotides that are formed complementary to one strand of DNA  Ribosomal RNA (rRNA)  Associates with proteins to form ribosomes in the cytoplasm  Transfer RNA (tRNA)  Smaller segments of RNA nucleotides that transport amino acids to the ribosome

 Through transcription, the DNA code is transferred to mRNA in the nucleus.  DNA is unzipped in the nucleus and RNA polymerase binds to a specific section where an mRNA will be synthesized. Transcription

 The code on the DNA is interrupted periodically by sequences that are not in the final mRNA.  Intervening sequences are called introns.  Remaining pieces of DNA that serve as the coding sequences are called exons. RNA Processing

 Experiments during the 1960s demonstrated that the DNA code was a three-base code.  The three-base code in DNA or mRNA is called a codon. The Code

 In translation, tRNA molecules act as the interpreters of the mRNA codon sequence.  At the middle of the folded strand, there is a three-base coding sequence called the anticodon.  Each anticodon is complementary to a codon on the mRNA. Translation

 The Beadle and Tatum experiment showed that one gene codes for one enzyme. We now know that one gene codes for one polypeptide. One Gene—One Enzyme

 Ability of an organism to control which genes are transcribed in response to the environment  An operon is a section of DNA that contains the genes for the proteins needed for a specific metabolic pathway.  Operator  Promoter  Regulatory gene  Genes coding for proteins Prokaryote Gene Regulation

The Trp Operon

The Lac Operon

 Controlling transcription  Transcription factors ensure that a gene is used at the right time and that proteins are made in the right amounts  The complex structure of eukaryotic DNA also regulates transcription. Eukaryote Gene Regulation

 Hox genes are responsible for the general body pattern of most animals. Hox Genes

 RNA interference can stop the mRNA from translating its message. RNA Interference

 A permanent change that occurs in a cell’s DNA is called a mutation.  Types of mutations  Point mutation  Insertion  Deletion Mutations

 Substitutions also can lead to genetic disorders.  Can change both the folding and stability of the protein Protein Folding and Stability

 Can occur spontaneously  Chemicals and radiation also can damage DNA.  High-energy forms of radiation, such as X rays and gamma rays, are highly mutagenic. Causes of Mutation

 Somatic cell mutations are not passed on to the next generation.  Mutations that occur in sex cells are passed on to the organism’s offspring and will be present in every cell of the offspring. Body-cell v. Sex-cell Mutation